1. Field of the Invention
The present invention relates to brush seals for dynamic shaft sealing, and in particular to brush seals having a deformable rear support.
2. Description of the Related Art
The background art will now be explained with reference to FIGS. 1 and 2. Brush seals are a common way of providing a seal to control leakage between a rotating shaft 2 and housing 4. They are typically found in gas turbine engines and the like, where they seal between a low-pressure region 6 of the engine and a high-pressure region 8. Brush seals usually include an annular rim or head portion 10 that is mounted on the housing 4 in such a way as to be coaxial with the shaft 2. The head portion 10 is usually formed from a high-grade stainless steel. A large number of bristles 12 extend from the head portion 10 towards the shaft 2 such that the ends of the bristles wipe against the surface of the shaft. The bristles 12 may comprise steels, nickel-based alloys or ceramics and may be welded into or clamped in the head portion 10, as appropriate. Individual bristles are relatively flexible, but collectively impede fluid leakage therepast. However, because of the pressure difference between the low-pressure and high-pressure regions of the engine, the bristles 12 have a tendency to bend towards the low-pressure side 6 of the brush seal. If the bristles 12 bend then the ends of some or all of the bristles may not continue to wipe against the surface of the shaft 2 and the leakage past the seal is no longer adequately controlled.
To stop the bristles 12 from bending towards the low-pressure side 6, the brush seal includes a rigid annular side support or backing plate 14. The support 14 extends from the head portion 10 towards the shaft 2 adjacent the bristles 12 on the low-pressure side 6 of the brush seal. Ideally the clearance between the support 14 and the surface of the shaft 2 should be as small as possible. However, if the shaft 2 is subject to excessive radial movement in the vicinity of the brush seal then the bristles will be deflected and the shaft may come into repeated contact with the support 14. This small clearance is not necessarily a disadvantage because it minimizes fluid leakage between the support 14 and the shaft 2. However, repeated rubbing contact increases the radial clearance between the support 14 and the shaft 2 due to deformation of the support. The support is thicker in the radial direction than in the axial direction and is therefore very resistant to deformation under compressive stress in the radially outward direction. Consequently, to accommodate radial deformation, the radially inner part of the support 14 plastically deforms by spreading out in the axial direction. FIG. 2 shows a rear support 14 that has been radially and axially deformed by repeated rubbing contact with the shaft 2. The axial deformation of the radially inner part 16 of the support 14 causes the bristles 12 to bend towards the high-pressure side 8 of the brush seal. This allows a gap 18 to open up between the ends of the bristles 12 and the surface of the shaft 2 which, together with the increased radial clearance between the shaft 2 and the support 14, reduces the sealing efficiency.
One way of preventing material in the base of the support from being axially deformed is to use a sacrificial rear support. A brush seal incorporating a sacrificial support is disclosed in EP-0453315-A1 (Cross Manufacturing Company Limited). In this case the base of the support is formed from an abradable material having a typical composition of 85% nickel and 15% graphite. The support is finished to define an inner diameter very slightly larger than the diameter of the shaft. If during operation of the brush seal the shaft contacts the base of the support then the abradable material will wear away instead of being axially displaced. This means that the bristles will not be displaced sideways by axial spreading of the support and the sealing efficiency will be maintained.
The present invention aims to provide an alternative solution to the problem that does not require the use of a sacrificial rear support.